Recovery of 2-chlorophenol from aqueous solutions by emulsion liquid membranes: batch experimental studies and modelling

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Abstract

A study of 2-chlorophenol recovery by emulsion liquid membranes was undergone. The effect of several permeation variables such as membrane phase composition, internal reagent concentration, solute concentration, permeation stirring speed, temperature, permeation time and phase ratios between membrane phase and internal phase and also between external phase and internal phase on 2-chlorophenol permeation was studied. The most relevant of these parameters were permeation stirring speed, temperature and phase ratio between external phase and internal phase. Advancing front model was applied to these parameters and described the experimental results.

Introduction

Chlorophenols are hazardous phenolic compounds that can be obtained from phenols by direct chlorination. They not only give water an unpleasant smell and flavour (that can be felt in a slightly phenol contaminated water when chlorine disinfecting treatment is applied in municipal plants prior to domestic use), but they are also toxic to several aquatic life forms even in very low concentrations. Besides this, chlorophenols are known as precursors of highly deleterious substances to the environment, such as certain polychlorodibenzo-p-dioxins (PCDD’s), polychlorodibenzofurans (PCDF’s) and polychlorophenoxyphenols (often generally called dioxins), which can be produced by their thermal or chemical degradation [1]. So recovery and reuse methods for the treatment of effluents containing chlorophenols (instead of destructive methods such as chemical oxidation and incineration) should be urgently developed. In this paper emulsion liquid membranes application was tried out in order to recover and concentrate 2-chlorophenol (since it is the major reaction product formed by direct chlorination of phenol with sodium hypochlorite) [1].

Li [2] firstly proposed emulsion liquid membranes technology for the separation of hydrocarbons. From then on, several applications have been developed for the removal and recovery of several solutes from either aqueous or organic solutions, including biological and medical ones [3]. In this technology, solutes are not only removed, but also concentrated. A schematic diagram of this technology is presented in Fig. 1. The effluent to be treated is contacted with an emulsion dispersed in globules (with diameters in the 0.1–2 mm range). Each emulsion globule consists of droplets of an aqueous internal receiving phase (with diameters in the 1–10 μm range) encapsulated in an organic membrane phase containing a surfactant. The referred emulsion is previously prepared by stirring at high mixing speed (emulsification). During the contact between wastewater and emulsion (permeation), solute transport occurs through the membrane phase into the internal receiving phase, where it is concentrated. Thus extraction and re-extraction (stripping) are carried out in a single step. After the subsequent separation by gravity of the effluent from the emulsion (settling), splitting of the emulsion (demulsification) is carried out. Demulsification is usually performed by applying an electrical field, in order to separate the phases that compose the emulsion: the internal aqueous phase and the organic membrane phase. At the end of the process, the membrane phase can be reused and the receiving phase — enriched in the recovered solute — can be recycled or recovered for solute. According to its composition, the effluent may be submitted to additional treatment processes or simply discharged.

Five industrial plants of emulsion liquid membranes were implemented up to now [4]. One of them was applied to the treatment of phenolic wastewater from a plastic factory in People’s Republic of China. Besides this industrial application, several batch studies with phenols are known. The majority has been performed with synthetic aqueous solutions of phenol [5], [6], but there are also studies using other phenols, such as cresols and nitrophenols [7], [8], [9]. Some references to chlorophenols removal preliminary tests by emulsion liquid membranes appear in literature [9], [10], but up to now no complete study of the most important effects has been published.

Section snippets

Theory

Since the beginning of its development, the modelling of permeation step of emulsion liquid membranes has been a major concern. Cahn and Li [5] were the first researchers to propose a rough and simple model for phenol permeation. They established that phenol flux through membrane phase was directly proportional to the difference between phenol average concentrations in the internal and external phases. The major limitation of this simple model was that the proportionality constant changed with

Experimental

Emulsions were prepared by emulsifying NaOH (Merck) aqueous solutions (internal phase) in an organic membrane phase composed by a paraffinic solvent (ShellSol T, Shell Chemicals Ltd.) and a non-ionic surfactant (polyamine ECA4360, Exxon Chemical Co.). A rotor-stator type high speed rotative disperser (IKA Ultra Turrax T50 with an IKA G45ff ultrafine turbine) was used at 116.7 s−1 with a residence time of 900 s. Permeation was made by dispersing this emulsion in the external aqueous phase

Estimation of physical properties

For practical use of the model, relations are needed for two model parameters. The first one is the distribution coefficient at equilibrium (α) for the 2-chlorophenol between the external phase and the emulsion after complete reaction of the internal phase reagentα=C/CIIIIt can be obtained from the experimental distribution coefficient (α′) for the 2-chlorophenol between the external and membrane phases at equilibrium. SinceCII=α′CIIIatr=Rand considering thatC=VICI+VIICIIVI+VII=VI/α′+VIIVI+VIIC

Results

Table 1 resumes viscosity data of internal and membrane phases and distribution ratios of 2-chlorophenol between aqueous and solvent phase at different temperatures. The high distribution ratios that were obtained explain the higher removal of 2-chlorophenol from external phase obtained (>99% in most tests). ShellSol T average molecular weight (169 kg/kmol [21]) was also used as a membrane phase molecular weight estimate.

The effect of several permeation variables such as membrane phase

Conclusions

A study of 2-chlorophenol recovery by emulsion liquid membranes was undergone. The effect of several permeation variables such as membrane phase composition, internal reagent concentration, solute concentration, phase ratios, stirring speed and temperature along time on 2-chlorophenol permeation was studied. Advancing front model was applied to the significant effects and it described 2-chlorophenol permeation experimental data reasonably. This was due to the high irreversible character of the

Acknowledgements

The authors would like to thank Shell Chemicals Ltd. and Exxon Chemical Co. by the reagent samples offered to this work. The financial support of FCT (Portuguese Government Institution) through PRAXIS XXI Programme is also acknowledged.

References (21)

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